The biggest risk management worries about nanotechnology, experts say, are the unknowns. Despite scores of ongoing studies, the human health and environmental effects of nanomaterials used in hundreds of consumer and industrial products still are unclear.

This uncertainty — and hints of possible toxicity of some particles in studies — has raised the specter of potential liability exposures ranging from consumer class action and directors and officers liability suits to mass tort litigation.

Businesses are under pressure to produce products that take advantage of nanomaterials' superior characteristics, but “what the potential exposure could be in the future is unknown,” said Peter Dion, product liability line of business director with Zurich North America in Schaumburg, Ill.

“The unknown and how to manage the unknown” are risk managers' biggest challenges, said Robert Gaus, senior vice president with Marsh Risk Consulting in Cleveland. “We are just not sure (about the exposure) from an industry perspective or a regulatory perspective.”

Nanoparticles are defined as being less than 100 nanometers, or billionths of a meter, in a given direction; 1 inch equals about 25.4 million nanometers.

Researchers have created hundreds of different nanomaterials from scores of elements and compounds from aluminum to zinc oxide. The most commonly used particles are nanosilver, which has antimicrobial properties and is used in sportswear and cosmetics; various nanocarbon materials, used in electronics and to strengthen composite materials; and nanotitanium dioxide, which transforms UV light into heat and is used in sunscreens.

Worldwide, there are 2,304 nanotechnology businesses — 1,197 in the U.S. — including manufacturers of raw nanomaterials, nano-enabled consumer and industrial products, instruments and medical and pharmaceutical products, according to Nanowerk L.L.C., a Honolulu-based news and information website.

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Studies of some nanomaterials have raised concerns about possible adverse health and environmental effects. Animal studies have found, for example, that filament-like carbon nanotubes can be inhaled, lodge in the lungs and cause inflammation, findings that have triggered comparisons to asbestos. No research has yet shown that carbon nanotubes cause asbestos-like disease, however.

Nanosilver, meanwhile, has been found to be toxic to beneficial soil bacteria, suggesting that wide dispersal could cause environmental damage.

Scores of university and government research projects into environmental and health effects are under way, some of them through the U.S. Environmental Protection Agency, the U.S. Food and Drug Administration and other government agencies.

The EPA and FDA have faced criticism, though, for not sufficiently regulating the use of nano-scaled materials. At the end of 2011, for example, the EPA's own inspector general reported that it relied too much on industry-submitted data and lacked the ability to compile and disseminate information about nanomaterials.

EPA has proposed a blanket rule requiring 90 days' advance notice from companies planning to manufacture, import or use nano-scale substances to allow time for a health and environmental impact assessment. For now, though, the agency continues to review materials on a case-by-case basis, said John C. Monica Jr., a partner with Porter, Wright, Morris & Arthur L.L.P. in Washington, who writes about nanotechnology law.

The FDA last year issued draft guidance to companies for evaluating the safety of nanomaterials used in food additives and packaging and cosmetics. In addition, the National Institute for Occupational Safety and Health, has issued recommended workplace exposure limits for carbon nanotubes and nano-scale titanium dioxide.

Overall, “regulation has lagged behind research and development a little bit,” but has picked up in the past two years, said Clayton Shoup, workers compensation line of business director with Zurich in Schaumburg.

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No nanotechnology liability lawsuits have been filed to date, but the lack of clear understanding of the possible effects of various nanomaterials has created concern about possible future exposures, insurance and legal sources say.

The most dire of these may be mass tort litigation involving alleged worker or consumer injuries from exposure to hazardous nanomaterials. Carbon nanotubes, with their physical similarity to asbestos fibers, have been most frequently cited as potentially presenting this risk. NIOSH also has identified titanium dioxide as a potential carcinogen, and the FDA is evaluating the safety of nano-scale titanium dioxide and zinc oxide in sunscreen products, agency documents show.

In the consumer products arena, carbon nanoparticles often are embedded in polymers or resins and thus present less risk than aerosolized particles that may be contained in health and beauty products, Mr. Gaus said.

Besides triggering personal injury suits, products containing allegedly dangerous nanomaterials also could give rise to medical monitoring claims, he added.

Tort actions would face legal hurdles, including proving that an injury was caused by a specific toxic material, legal sources say; asbestos cases didn't face this hurdle because asbestos triggers a signature disease, mesothelioma. Still, state courts have more lenient standards for causation testimony than federal courts, and suits still might be filed there, said Catherine Morris Krow, a partner with Orrick, Herrington & Sutcliffe L.L.P. in San Francisco.

Some doubt the possibility of an asbestos-like mass tort crisis, though. In part, this is because — unlike asbestos — nanomaterials are the targets of early and widespread toxicology study.

“I just don't see it happening,” Mr. Monica said.